Cartonboards made from cellulosic fibers are often made of three or in some cases four plies. Typically, the top and back plies are made from chemical pulp. Mechanical pulps such as groundwood, pressure groundwood, thermomechanical pulp (TMP), chemimechanical pulp (CTMP), alkaline peroxide mechanical pulp (APMP) and machine broke are often used in the middle ply or plies of the cartonboard. The mechanical pulp or low refined chemical pulp is used in the middle ply in order to give the highest possible bulk. Also, basis weight of the top and back plies is minimized in order to achieve high bulk and also for decreasing raw material costs.
Cartonboard can be used for production of folding board. A basic requirement for a folding board is a certain level of mechanical strength and stiffness. Bending stiffness for certain smoothness levels, and especially cross machine direction bending stiffness, is critical. Bending stiffness is affected mainly by thickness of the board and modulus of elasticity given by the raw material in it. The optimal structure is when the middle ply has high bulk and the top and back plies have high modulus of elasticity. The lower the basis weight that can be achieved at a given stiffness, the better the yield. When this is fulfilled, more area, i.e. paper or board, can be produced from the same weight of pulp.
When the board is creased, there are tensile, compression and shear forces acting on the board. Cracking of the board surface should be avoided and the cracking tendency is also affected by the board structure. In order to minimize the cracking tendency, the stretch to break of the top ply should be as big as possible. Also, z-direction strength is important. If the z-strength is too low, the board can delaminate during printing operations. On the other hand, if the z-strength is too high, cracking can occur since the stretch is too big for the top ply if the middle ply does not delaminate in creasing.
The forming section of a board machine typically provides separate forming of individual plies with fourdrinier rolls. A middle-ply fourdrinier is often equipped with a top dewatering unit in order to improve formation and increase drainage capacity.
Fines and filler distribution of the sheet in z-direction depends heavily of the dewatering of the wet web. It depends on, for example, if the dewatering is done in one direction (fourdrinier) or two directions (twin wire or MB-type of former). Furthermore, the side part of the web, which is where water was taken out, typically contains fewer fines, i.e. this side is washed clean from fines.
If several wet sheets are wet couched together, the weakest point of the board is typically between different plies due to low fines content and more open structure in this point. This leads to a situation where the board is delaminated during creasing of the different plies, thus causing imperfect crease which can lead to cracking during the converting operations.
In the prior art there are several ways to handle these problems.
One way is by increasing the top and bottom ply grammage. In this way, top and bottom ply strength is increased which thus will prevent cracking tendency. The disadvantage with this is that cost increases due to increased chemical pulp usage and reduced bending stiffness index, i.e. the structure is not optimized seeing to bending stiffness.
Another way is by increasing refining of the pulp in the middle ply. Increased refining increases the amount of fines present which leads to increased amount of fines located between the different plies. However, this unfortunately reduces the caliper of the board and thus also reduces the bending stiffness index.
Yet another way is by decreasing the amount of water taken from the middle ply top dewatering unit. This results in that fewer fines are taken from the middle ply. However, it unfortunately decreases formation.
The most commonly used way to solve the problems discussed above, is to spray starch between the plies before couching the plies together. Unfortunately, starch forms very un-stretching bonds between the plies which increases cracking tendency of the board.
There is thus a need for an improved process for the production of a multi-ply board with decreased cracking tendency at the same time as delamination of the plies of the board is avoided or reduced.